I have a feeling this more about control and feeling like she has won than the tea and cokes. If she gets you to do what she wants, she wins. She gets to feel like the status quo of your original household is returned to balance, you haven't grown up, she hasn't gotten older and she is the pampered mother again. And she knows exactly what she's doing and that it is annoying/upsetting to you, otherwise she wouldn't pull the bait and switch tactics.
...My problem isn't being asked to help, or being asked to look after myself regarding refreshments, it's feeling "tricked" and manipulated and as if I've been used to host for her/get something she wants under the guise of giving me something. If I were asked directly to help or get a drink for her, that would be perfect and I'd be happy to! My goal is not to avoid doing any work, just to be asked respectfully
I totally get where you're coming from. My late grandmother used to do this to all of her children and grandchildren. What I resented was the fact that she thought she was outsmarting me or one-upping me. For example, she'd invite you over, you'd say yes, and then she'd say "On your way over, stop at the store and buy me X." I was a little girl, she was healthy and able-bodied and in her 50s and I remember her doing it to my family. We'd be all dressed in fancy clothes and on our way to her house for dinner and we'd all have to stop at the grocery store to buy her toilet paper, while dressed up.
When I was an adult, if she'd invite me to something, DH (then, boyfriend) would say "Oh, that's nice." And I'd tell him "No, we have to say no, because for that 1 hour of X event, we're going to spend an extra 5 hours helping her do A, B, and C. She will suggest that we go to lunch. She won't pay, so we'll have to buy her lunch. At a restaurant she chooses. And we'll be running other errands for her later, etc..."
It was like playing a chess game. You couldn't take anything at face value. You had to think 5 moves ahead to prevent being check-mated. You had to imagine all the different ways that grandma was going to thwart your attempts at just having a short, nice visit. And imagine all the different favors she was going to ask. Then, that ended up controlling what you wanted to do. Maybe you wanted to drive the 4-seater car that day, but you figured she might spring a few extra guests on you. "Oh, you don't mind giving a ride to these people, do you?" So you have to plan to take the 2-seater car so it's just you and grandma, for example.
OP, in your case, you end up having to drink 1 liter of water before each visit with your mother so that you don't go thirsty because you plan to decline each and every drink she offers you. Then, you're still being controlled, because who wants to drink so much ahead of time, planning not to drink later.
I think it is a power play. I think she gets some satisfaction from making you "lesser" or act like a servant. Maybe you have achieved a higher level of education than she has and she's jealous, for example. So she feels superior when she does this.
I think you can address it head on, politely, like a PP suggested. But if this is a power play, as I suspect, your mother will deny it. She'll tell you that you're too sensitive and you need to stop overreacting. And that she's not asking that much.
After you've addressed it politely and honestly a couple of times, then I'd draw firmer boundaries.
In your shoes, I would decline. Whenever she says "Can you feed the cats?" I'd reply "No. Sorry. I'm here for a visit. Not to feed your cats." And if she argues, I'd say "They're your responsibility" and then walk over to your boyfriend or other guests and join their conversation. If she says "Fix me a Coke, too" I'd say "No thanks." That'll confuse her. Then onlookers think you're still being polite because you threw the word "thanks" in there. And if they do join her bandwagon and say you're being harsh, you can then say to them "Then, you fix her the Coke." And after a few interactions like that, end the event early and leave. Say "you invited me here. Clearly, it was not for dinner, it was for me to serve you. Have a nice evening. Bye." And leave.
And then only meet her at restaurants or the mall or neutral locations for a while.